(1) Field of the Invention
This invention relates to clad metal sheet intended for the manufacture of tubing and headers used for conveying water, e.g. as used in heat exchangers such as automobile radiators. More particularly, the invention relates to such clad metal sheet structures and the tubing or headers made therefrom.
(2) Description of the Related Art
Water-conveying metal tubing used in heat exchangers is often subject to internal corrosion from the cooling water, especially when such water contains various chemicals, such as those used to elevate the boiling point or to depress the freezing point of the coolant, etc. The metal used for such tubing is frequently an aluminum alloy, and alloys of the AA3000 series (containing Mn) are often chosen for their formability, heat conduction and relatively low cost. It is conventional to provide one surface of a sheet of such alloy with a coating of a so-called brazing alloy, i.e. an aluminum alloy having a low melting temperature. This is often an alloy of the AA4000 series containing large amounts of Si. The surface of the metal sheet coated in this way is the surface intended as outer surface of a tube made of the metal sheet. This allows the tubing, once formed, to be connected by brazing to metal fins intended to provide enhanced heat exchange with air. Such cladding is often referred to as “air-side cladding” for obvious reasons.
The surface intended to be the internal surface of the tube is sometimes coated or clad with a metal intended to reduce or delay corrosion. The internal fluid in a heat exchanger (e.g. radiator) would normally and ideally contain 50% de-ionised or de-mineralised water, with about 50% inhibited coolant added. However, despite such benign coolants, corrosion still occurs, especially if incorrect filling practices are carried out. An internal cladding of the core of this kind is often referred to as a “water-side cladding.” At the moment, alloys such as AA7072 and AA3003 (with an addition of 1.5 wt % Zn) are commonly used to provide such protection. Nevertheless, alloy AA7072 is soft and can be damaged by erosion in service. Due to its low strength, AA7072 also lowers the overall strength of the tube in pre- or post-braze conditions.
U.S. Pat. No. 7,387,844 issued to Ueda et al. on Jun. 17, 2008 discloses a brazing sheet having a structure of the kind indicated above. The water-side cladding contains 2 to 9 wt % Zn and various other alloying elements.
PCT patent publication WO 2007/042206 to Vieregge et al. published on Apr. 19, 2007 likewise discloses a multi-layer brazing sheet intended to provide long life corrosion resistance. The water-side cladding includes a braze cladding of an AA4000 series alloy and, between the core and the inner braze cladding, there is an interliner of an AA3000 series alloy or an AA1000 series alloy. For example, the interliner may be an AA3000 alloy containing less than 0.25 wt % Cu, 0.5-1.5 wt % Mn, less than 0.3 wt % Mg, and 0.1-5.0 wt % Zn. While this alloy layer is described as an “interliner” beneath the internal brazing alloy, it should be noted that the internal brazing alloy disappears during the brazing step, in this case forming a filler metal to form interior tube compartments. In use, therefore, the interliner itself forms a coating that is directly exposed to the cooling water.
European patent publication EP 1 939 312 A1 to Minami et al., published on Jul. 2, 2008 discloses a clad member comprising a core, an outer skin on one surface of the core and an inner skin provided on the other surface of the core layer via an intermediate layer. The inner skin layer is an Al—Si brazing material that disappears after a brazing step. The interlayer then becomes the layer exposed to cooling water to provide corrosion protection. In this regard, the subject matter of Minami et al. is similar to that of Vieregge et al. described above.
While clad structures of this kind are useful in reducing internal corrosion of tubes used in heat exchangers, corrosion and erosion still takes place and it would therefore be advantageous to provide metal structures that have enhanced resistance to such corrosion and erosion.